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Old 01-18-2014, 05:06 PM   #1
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structural strength advice needed


I have a kind of unusual project that I am planning and need some advice. I have a super single waterbed that I want to elevate so that I can create a large storage area underneath.

The mattress is 48"x84"x8". Once you've done the conversions involved, this equates to 1120 lbs of water. The waterbed currently sits on top of a 1/2"x4'x84" piece of OSB, which sits atop a 1/2" OSB boxed frame with some diagonally positioned cross pieces of 1/2" OSB.

My plan is to place the mattress on a 1/2"x4'x84" piece of plywood. Supporting the plywood will be (7) 4"x4"x4' posts placed on 14" centers across the width of the bed. My plan is to then use (2 or 3) 4"x6"x84" Douglas Fir posts placed lengthwise under the 4"x4" posts. I would then stand 4"x4"x5' posts under the corners and one on each side in the middle (if necessary, based on your input).
I plan to use other lumber (2x4s, etc.) to tie all of the framework together so that it doesn't move.

My questions are these. Does this sound like enough support for this kind of weight? Can I get away with using just 2 of the 4x6s or do I need the third one down the center lengthwise? Are the 4x6s on the outer edges stout enough so that I don't have to add the center support lengthwise at 24"?

Ultimately, I want to have as much access to the storage area as possible without having posts in the way. A free open space with only 4 corner posts would be ideal, but not sure if I can get away with it.

Any/all advice/input/recommendations or changes to this plan will be greatly appreciated. Thank you for your time and consideration. Rob

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Old 01-18-2014, 07:12 PM   #2
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structural strength advice needed


http://www.buildawaterbed.com/basics.htm

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Old 01-18-2014, 08:57 PM   #3
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structural strength advice needed


I appreciate your response, but the link describes how to build a waterbed pedestal like the one that I already have. I want to elevate the bed 4-5' above the floor, unlike the factory pedestal which is under 1' and allows no room for storage. I was thinking that there is some way to calculate weight per span for a particular type and size of lumber. Any ideas on that?

My profile lists me as living in MI; south central MI if that helps any.
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Old 01-18-2014, 09:21 PM   #4
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structural strength advice needed


I think you need to bring in a structural engineer. A frame to support almost 1200 pounds of water. Plus the normal waterbed frame. Along with the wood weight of what you describe, and yourself and another person in the moving in the bed. Will cause weight stress/shear you are not allowing for. The higher the frame, the greater the shear.
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Old 01-19-2014, 09:59 AM   #5
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structural strength advice needed


I would build it exactly like a free standing deck with adequate angle bracing and a 2x8 floor system with plywood decking. you could do four posts, one on each corner but will have to go under house and block out the floor system where the posts land to give the posts under support.
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Old 01-19-2014, 02:56 PM   #6
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structural strength advice needed


Assuming 2000lbs maximum load for the sake of argument, the structure you describe (ie 4x4s crossways supported by 4x6s lengthways would be fine to support the load in terms of bending stress in the timber.

The two problems you will have are;

1. At 5ft-height, ensuring that your structure is stable and suitably restrained by adequate diagonal bracing, firmly fixed and;

2. ensuring that you have sufficient support below the legs, with each leg possibly supporting up to 500lbs.
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Old 01-19-2014, 03:20 PM   #7
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structural strength advice needed


If your floor on a concrete slab?
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Old 01-19-2014, 06:56 PM   #8
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When you (hand drive) say to use adequate angle bracing, are you talking about cross bridging between the 2x8s, between the support posts, or both? The bed will be in the basement upon a concrete floor so I would think there would be adequate load bearing capacity in that respect.
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Old 01-19-2014, 08:20 PM   #9
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structural strength advice needed


Quote:
Originally Posted by killermrob83 View Post
I have a super single waterbed that I want to elevate so that I can create a large storage area underneath.
You don't mention what type of storage. Do you want it all open so you can see it all or do you want to conceal what's being stored?

That little detail makes a big difference as far as design.
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Old 01-19-2014, 10:14 PM   #10
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structural strength advice needed


I would like it all open so that I can store rather large objects underneath, like say a quadrunner.
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Old 01-19-2014, 10:24 PM   #11
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structural strength advice needed


Then you better anchor the support post.
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Old 01-20-2014, 09:48 AM   #12
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structural strength advice needed


Quote:
Originally Posted by killermrob83 View Post
When you (hand drive) say to use adequate angle bracing, are you talking about cross bridging between the 2x8s, between the support posts, or both? The bed will be in the basement upon a concrete floor so I would think there would be adequate load bearing capacity in that respect.
45 degree bracing at the top of structure. also, if you place point loads onto the slab ( post placement) you will want to spread the load with structure on top of the concrete. leave one side open for access and then build a double 2x8 the other three sides and set the posts on the double 2x8. this eliminates the 3 1/2" by 3 1/2" tiny footprint point load from the posts
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Old 01-21-2014, 07:56 PM   #13
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structural strength advice needed


At this point, I am still leaning toward using the 4x4s and 4x6s for the structural base, although I am definitely considering building it like a free standing deck, all depending upon the number of support legs I will have to use. As aforementioned, I would like to only have to use a leg in each of the corners.

This question is for tony.g. If I should happen to go with the 4x4s and 4x6s crosswise and lengthwise respectively, will I still be within the bending stress parameters of the timber if I only use corner posts for legs?
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Old 01-22-2014, 09:13 AM   #14
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structural strength advice needed


If there are just two longitudinal timbers, assume each would carry one-half the total load, say 1000lbs each.

The maximum bending moment (M) in the beam would be W*L/8 = 10500 lbsin.

Assume the actual dimensions of the beam are 3.5" x 5.5" approx (?),

then the bending stress = M/Z, where Z = breadth x depth squared/6

so bending stress = 10500/ (3.5 x 5.5 x 5.5/6)

= 595 lbs/sq.in.

Most structural timbers will easily meet this - google bending stress of timber.
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Old 01-22-2014, 10:17 AM   #15
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structural strength advice needed


Your technical expertise is greatly appreciated.

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